The goal of this project is to better understand the renal concentrating mechanism. Electron probe analysis of frozen hydrated bulk kidneys will allow us to measure the renal concentration gradient of Na, K and Cl directly along the longitudinal cortico-papillary axis. Analysis of frozen hydrated bulk kidneys will provide the composition of tubular fluid in outer and inner medullary segments of the nephron, inaccessible to sampling by micropuncture. Microfluorescence urea determination of renal thin frozen slices will provide the cortico-papillary urea gradient at high resolution. We will attempt measuring the gradient profile of sorbitol accumulation in antidiuresis. Analysis will be performed in three types of mammals: rats, rabbits and psammomys, each of which may have a different renal concentrating mechanism. Analysis will performed in extreme diuretic states: Water deprivation, osmotic diuresis, and, in the rat, water diuresis by water load. Understanding of the renal concentrating mechanism is stalled by the lack of new data or by the reliance on old data which do not provide enough resolution. With gradient profile and composition of intratubular fluid in the outer and inner medulla, we will be able to evaluate ours and other models of renal concentration in the inner medulla.